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Thyroid Uptake Neck Phantoms are Not Created Equal

Yale University School of Medicine, New Haven, Connecticut
Freedman and Associates, New Haven, Connecticut

ABSTRACT

The main purpose of this communication is to alert nuclear medicine departments to the fact that the earlier version of the water phantom grossly overestimates soft-tissue attenuation in the neck, resulting in calculated thyroid uptake values which are significantly overestimated (in hyperthyroid patients we noted uptake values approaching or exceeding 100%). We believe that the solid Lucite phantom (which is the one recommended by IAEA) better approximates the human neck soft tissue overlying the thyroid. Institutions that continue to use the water phantom should be aware that their thyroid uptakes will be relatively elevated and the normal range must be shifted accordingly. Our normal range is 10%–30% uptake of ¹²³I at 24 hr for the Lucite phantom. For the water phantom, the estimated normal range would be 15%–45%.

In addition, the phantom type should be considered when comparing uptake results with those from another institution for a particular patient. Also, treatment doses for Grave's disease could be significantly affected, if such doses are calculated by a formula that depends on uptake.